I don't object to it, so in the spirit of things, I will put forward
strawman reasons for maintaining the status quo. It's a poor argument
that won't point both ways.

1) It would be politically difficult to allocate 50 or more codepoints
on the BMP for a dead language when many living languages are not yet
supported there. (24 Greek letters plus at least one more Coptic-unique
letter, plus who knows how many more, times 2 for upper/lower case.)

2) It would be technically obnoxious to have some Coptic letters on the BMP
and others on the Astral Planes. "Either you're on the bus, or you're off
the bus." (The Merrie Pranksters, dated U.S. cultural reference.)

3) It would be suicidal to move the existing Coptic letters to the
Astral Planes.

4) Some folks may have already encoded Coptic text using the Greek
unification, and dis-unification would make their data putatively
invalid (a new implementation would make it come out as Greek).
This was the argument used for not unifying the EURO SIGN with
the EURO-CURRENCY SIGN.